Ion implantation is a process of depositing chemical species into a substrate by direct bombardment of the substrate with energized ions. In semiconductor manufacturing, ion implanters are used primarily for doping processes that alter the type and level of conductivity of target materials. A precise doping profile in an integrated circuit (IC) substrate and its thin-film structure is often crucial for proper IC performance. To achieve a desired doping profile, one or more ion species may be implanted in different doses, at different energy levels, and sometimes at different incident angles.
For a uniform distribution of dopant ions, an ion beam is typically scanned across the surface of a target wafer. FIG. 1a shows a typical beam path 10 formed by scanning an ion beam spot 102 across a wafer 104. The ion beam spot 102 may be scanned back and forth in the X direction while the wafer 104 is translated in the Y direction, thereby forming the zigzag pattern of the beam path 10. FIG. 1b shows another typical beam path 11 formed by scanning the ion beam spot 102 across the wafer 104. Here, the ion beam spot 102 is scanned along straight lines in the X direction and makes square turns at the end of a sweep.
As the semiconductor industry is producing devices with smaller and smaller feature sizes, ion beams with lower energies are required for wafer implantation. Compared with high- or medium-energy ion beams, low-energy ion beams present some unique challenges. For example, a low-energy ion beam usually has a large beam spot, which can cause problems for beam utilization and uniformity tuning. In addition, the shape or current density of a low-energy ion beam can change substantially as it propagates down a beam line. As a result, if different portions of a wafer meet the low-energy ion beam at different positions along the beam line, different dopant profiles may be created for the different portions of the wafer. This problem is illustrated in FIG. 2, wherein an ion beam 202 is propagating along the Z direction. If a wafer 204 meets the ion beam 202 at a first position Z=Z1, the wafer 204 will see a beam spot 206. If the wafer 204 meets the ion beam 202 at a second position Z=Z2, for example, when the ion beam 202 is scanned across the surface of the wafer 204, the wafer 204 will see a beam spot 208 that may be quite different from the beam spot 206. Therefore, the portion of the wafer 204 implanted with the ion beam at beam spot 208 may have a different dopant profile from the portion implanted with the ion beam at beam spot 206. The problem of beam-line variation is not unique to low-energy ion beams but can also be observed in some high-current ion beams.
As a countermeasure to the beam-line variation problem, a concept known as “isocentric scanning” has been proposed. Isocentric scanning generally involves moving a wafer with respect to an ion beam in such a way that the point in space where the ion beam strikes the wafer surface remains the same no matter which part of the wafer surface meets the ion beam. It should be noted that isocentric scanning does not have to cover the entire surface of a wafer but may cover only a portion of the wafer surface.
FIG. 3 illustrates isocentric scanning of a wafer 304. An ion beam 302 may propagate along the Z direction, and the wafer 304 may be translated within the X-Y plane to perform the isocentric scanning. Effectively, the wafer 304 is moved in such a way that a beam spot 306 (i.e., a cross section of the ion beam 302) having the same shape and current density distribution will strike different portions of the surface of the wafer 304 at a same point (X0, Y0, Z0) in space.
FIG. 3 only shows the simplest example of isocentric scanning wherein the ion beam 302 has a normal incidence on the wafer 304. These days, however, it is often necessary to implant a wafer at one or more different incident angles other than the normal incident angle, which technique is known as “angled ion implantation.” Sometimes, it may also be desirable to implant different portions of a wafer at different incident angles. In existing ion implantation systems, angled ion implantation is typically accommodated by maintaining an ion beam along a fixed reference direction and tilting a target wafer with respect to the ion beam. To achieve isocentric scanning on a tilted wafer, the wafer has to be moved in a complex pattern with respect to the ion beam.
FIG. 4 illustrates a complex movement of a tilted wafer 404 for isocentric scanning by an ion beam 402. The ion beam 402 propagates along the Z direction. For isocentric scanning, the wafer 404 is moved in such a way that the same beam spot (or ion beam cross section) 406 strikes the surface of the wafer 404. Since the wafer 404 is tilted with respect to the ion beam 402, the wafer 404 is moved not only in the X-Y plane, but also in the Z direction. For example, at position A, the center of the tilted wafer 404 may be at Z=ZA. At position B, the center of the tilted wafer 404 may be at Z=ZB (ZB≠ZA). The movement in the Z direction has to precisely coordinated with the movement in the X-Y plane in order to meet the isocentric scanning requirement.
To precisely control the tilting and the three-dimensional (3-D) translation of a wafer for isocentric scanning, existing ion implantation systems have to be equipped with a fairly complex end station to hold, tilt, and move the wafer. FIG. 5 illustrates a typical setup for isocentric scanning in existing ion implantation systems. An end station 508 may be a process chamber or similar component that houses a mechanism (not shown) for holding a wafer 504 and for controlling wafer movements. An ion beam 502 may propagate along a reference direction 50 and may be directed into the end station 508 via an aperture 510. With respect to the end station 508, the angle of the ion beam 502 is relatively fixed. Isocentric scanning is achieved by tilting and moving the wafer 504, all inside the end station 508. For example, the wafer 504 may be tilted by an angle θ so that the normal direction 52 of the wafer 504 is at an angle θ with respect to the reference direction 50. The mechanism in the end station 508 is capable of moving the wafer 504 in all three dimensions based on the tilt angle θ and the isocentric scanning requirement. Such capabilities necessitate a sophisticated design of the end station 508 that is often expensive to build and maintain.
In view of the foregoing, it would be desirable to provide a technique for isocentric ion beam scanning which overcomes the above-described inadequacies and shortcomings.